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Q u e s t i o n s a n d A n s w e r s
Calculate as follows:
Normal Full Load Current =
Nameplate Volt Amps
=
10,000 VA
Line Volts 480 V
= 20.8 Amperes
Maximum Short Circuit Amps =
Full Load Amps
=
20.8 Amps
4% 4%
= 520 Amps
The breaker or fuse would have a minimum interrupting rating of 520 amps at 480 volts.
Example: Determine the interrupting capacity, in amperes, of a circuit breaker or fuse required for a 75 kVA, three phase
transformer, with a primary of 480 volts delta and secondary of 208Y/120 volts. The transformer impedance (Z) = 5%. If the
secondary is short circuited (faulted), the following capacities are required:
Normal Full Load Current =
Volt Amps
=
75,000 VA
3 3 x Line Volts 3 3 x 480 V
= 90 Amps
Maximum Short Circuit Amps =
Full Load Amps
=
90 Amps
5% 5%
= 1,800 Amps
The breaker or fuse would have a minimum interrupting rating of 1,800 amps at 480 volts.
Note: The secondary voltage is not used in the calculation. The reason is the primary circuit of the transformer is the only winding being
interrupted.
21. Can Single Phase Transformers be used for Three Phase applications?
Yes. Three phase transformers are sometimes not readily available whereas single phase transformers can generally be found
in stock. Three single phase transformers can be used in delta connected primary and wye or delta connected secondary. They
should never be connected wye primary to wye secondary, since this will result in unstable secondary voltage. The equivalent three
phase capacity when properly connected of three single phase transformers is three times the nameplate rating of each single phase
transformer. For example: Three 10 kVA single phase transformers will accommodate a 30 kVA three phase load.
22.Does ACME provide “Zig-Zag” Grounding Transformers?
Yes. Please refer to Page 35 for a special diagram which can be used to connect standard single phase off-the-shelf transformers in a
three phase zig-zag manner. This system can be used for either grounding or developing a fourth wire from a three phase neutral. An
example would be to change a 480 V — three phase — three wire system to a 480Y/277 V — three phase — four wire system.
23. What color are ACME Dry-Type Transformers?
ASA 61 (NEMA) light gray is used on all enclosed transformers from .050 to 1000 kVA.